The present invention relates to refractory vessels, and more particularly, to materials for lining refractory vessels.
Black liquor is a by-product of the wood pulping process. Black liquor is a mixture of hydrocarbon, caustic, chlorine and other corrosive chemicals. It is normally completely combusted in a recovery boiler. Inorganic chemicals including sodium sulfate and sodium sulfide are recovered for reuse in the pulping process. Heat produced by the complete combustion is converted to steam, which in turn is used to produce process heat and/or electrical power. An alternative device proposed for recovering inorganic chemicals from black liquor is a gasifier. In a gasifier, the black liquor is burned in a sub stoichiometric atmosphere to produce a combustible gas. Inorganic salts are recovered in the process. The combustible gases can be used directly to fuel a gas turbine, or combusted in a power boiler.
Low pressure gasification requires an insulated environment, which is obtained through a refractory lined vessel. Refractory vessels of current design for use as gasifiers employ a stainless steel jacket and a fused-cast alumina liner. The alumina liner normally has a first inner layer of blocks comprising both alpha and beta alumina and a second outer layer of blocks comprising beta alumina. A small expansion allowance is provided between the outer layer of beta alumina blocks and the stainless steel jacket.
After vessels of this design are operated for a few months, it has been found that the refractory materials react with the soda in the liquor and expand to completely consume the normal expansion allowance provided between the refractory and the stainless steel jacket. At this point, the refractory layers begin to press against the inside of the stainless steel jacket. This situation causes early failure in the refractory materials themselves and plastic deformation of the stainless steel jacket. As a consequence, refractory linings of a conventional design have been unsatisfactory for use in a black liquor gasifier.
As a result of a study of a prior gasifier, it has been found that the alumina refractory material has not only been subject to thermal expansion, which was known in the prior art, but also is subject to chemical expansion. Chemical expansion is caused by sodium, present in the black liquor, combining with the refractory material to produce sodium aluminate. It has been found that sodium aluminate expands on the order of 130% relative to the original alumina. This causes both greater radial and vertical expansion in the refractory material as the vessel is used over time. This additional expansion of the refractory material presses outwardly on the interior of the stainless steel jacket and places it under stress. Chlorine and moisture present on the inner surface of the stainless steel pressure vessel, loaded in tension by the refractory expansion cause stress corrosion cracking of the stainless steel vessel, subjecting it to early failure. This internal stress caused by the thermal and chemical expansion of the refractory must be controlled to an acceptable level. Also, for the purpose of pressure vessel design as per ASME code, it is necessary to clearly and explicitly define the xe2x80x9csecondary stressxe2x80x9d due to the expansion of the refractory lining. Secondary stresses are all stresses not due to the internal gas pressure.
The present invention therefore provides a refractory liner for a vessel which accommodates the expansion of the refractory lining and provides a known secondary stress on the pressure vessel. The vessel has a cylindrical metal shell preferably having a hemispherical dome. A refractory shell has a cylindrical portion spaced inwardly from the metal shell and preferably a hemispherical dome portion spaced inwardly from the hemispherical dome of the metal shell. The refractory shell is sized to leave a uniform expansion gap between the liner and the shell in the cylindrical section.
In a preferred embodiment, the center of curvature of the hemispherical dome comprised of the refractory is at a lower elevation than the center of curvature of the hemispherical dome comprised of the metal shell. This provides an expansion gap which increases in thickness as the dome curves upwardly and inwardly. This crescent shaped gap in the dome allows for radial expansion of the refractory dome as well as axial expansion of the cylindrical section. The entire refractory dome rises in the vertical direction as the cylindrical section expands. A selectively crushable liner is positioned in the gap. The liner has a predetermined yield and crushing stress that will provide controlled resistance to expansion of the refractory shell. The resistance provided is significantly less than the yield strength of the metal shell while providing sufficient resistance to expansion of the refractory shell to allow controlled growth of the shell. Since the mechanical characteristics of the crushable liner are known, the internal secondary stress on the steel shell can be accurately described.